1. Field of the Invention
The present invention relates to a plasma enhanced atomic layer deposition (PEALD) apparatus and method of forming a thin film using the same. Particularly, the present invention relates to a PEALD apparatus and method of forming a thin film using the same, whereby a thin film is formed to an atomic layer thickness by periodically supplying time-divisional (sequential) combination of process gases to a reactor, during which plasma is generated adequately and synchronously with process gases on top of a substrate in order to accelerate the process reaction and facilitate more efficient reaction.
2. Description of the Related Art
As semiconductor integration technologies advance, process methods for depositing a thin film uniformly and conformally become increasingly important. Here, the thin film may be an insulator or a conductor. Thin film deposition methods are largely categorized into two types: chemical vapor deposition (CVD) and physical vapor deposition (PVD). In a CVD, gas phase materials generally react over the top surface of a substrate heated to a temperature of 100–1,000 degree. C., whereby a compound produced as a result of such reaction is deposited on the top surface of the substrate. On the other hand, the PVD method such as sputtering deposition or simply sputtering, are widely used, whereby the process takes place also in a vacuum state in a reactor. When a gas such as Ar gas as an example is supplied to a reactor, the Ar gas becomes positively ionized by a plasma and attracted to a target located inside the reactor. As the ionized Ar atoms get closer to the target, they get accelerated further and as the ionized and accelerated Ar atoms strike the target, the material of the target is scattered and deposited on the surface of a substrate, wherein the material of the target is deposited on the surface of a substrate without a chemical or structural change.
The advantage of a PVD is to make it possible to deposit an alloy or an insulator. Nonetheless the less CVD is more widely used since CVD has advantages over PVD, causing less damage to substrates on which a thin film is deposited, offering low thin film deposition cost, and capability of thin film deposition.
However, as the density of semiconductor devices recently continues to increase from micrometers to nanometers, conventional CVD methods do not perform satisfactorily in forming a thin film uniformly in thickness in nano scale on a substrate or achieving an excellent step coverage. In particular, in case of high aspect ratio, in turn large step difference, such as contact holes, vias or trenches, of small dimensions than micrometer (micron), difficulty exists in forming a thin film having uniform composition regardless of high aspect ratios in various patterns such as contacts, via holes or trenches over the entire surface of the substrate.
Unlike conventional CVD method, where all process gases are simultaneously supplied (in flow) or removed (outflow). In ALD method each atomic layer of thin film is formed by repeating the thin film deposition process by avoiding the direct contact of process gases on the substrate surface and by replacing the process gases rapidly and sequentially within the process gas cycles. This new method of forming thin films is being developed and used.
When the aforementioned atomic Layer Deposition method is used, the deposition takes place only by the material that is adsorbed on the surface of a substrate, i.e., only by the chemical molecule that contains the elements for forming a thin film, whereby a thin film is formed uniformly over the entire surface of a substrate regardless of the quantity of the process gas because the amount of adsorption is on the surface of a substrate is limited by the thickness of a mono layer. Therefore, a uniform thickness of thin film can be formed regardless of the location of the areas of high aspect ratio, in turn, large step difference, and even a thin film with the level of thickness of several nano meters can be formed, and also the thickness of the thin film can be controlled by adjusting the time-divisional combinations of the steps of the formation processes. Furthermore, it is possible to control the thickness of the thin film being formed because the thickness of the thin film formed by deposition during the process gas supply cycle is almost constant.
According to the conventional ALD method described above, in order to avoid mixing of gas materials in a gas phase supplied to a reactor in a time-division mode, remaining deposition gas or reaction gas molecules excluding those adsorbed on the substrate surface among deposition gas or reaction gas supplied to the reactor should be removed, for which the processes of vacuum evacuating the deposition gas or reactant gas from the reactor for several seconds or purging those gas out of the reactor by feeding an inert gas such as Argon gas, must be included in the cycle of gas supply and evacuation thereof.
Accordingly, use of a conventional CVD equipment, in which supply and removal of process gases cannot be made at a fast rate at the speed within several seconds, may increase the deposition time required for obtaining a thin film of a desired thickness because the longer time needed for replacing the process gas. Increased deposition time reduces the number of wafers to be processed per unit time by one equipment, thereby increasing the processing cost. As the deposition time increases, the number of wafers that can be processed with each equipment decreases, thereby, the cost of process increases. Therefore, it is necessary to reduce the deposition time in order to utilize ALD methods for producing semiconductor products using ALD methods.
FIGS. 1a and 1b show two schematic structure of a conventional chemical vapor deposition apparatus.
Referring to FIG. 1a as a first prior art of an Atomic Layer Deposition(ALD), at the top part of a reactor 100 gas inlet tube 120 that supplies process gases including deposition gas, reactant gas, purge gas is located, and on the side of the reactor 100, a gas outlet tube 122 for discharging the process gases from the reactor 100 is located. Inside the reactor 100, a substrate carrier 116 is located on which a substrate 110 is loaded. Inside the reactor 100, a showerhead 112 is located and at the top and in the middle of the showerhead 112, the process gas inlet tube 120 is connected. In the lower part of the reactor 100, a substrate carrier driver 118 is mounted. This substrate carrier driver 118 moves the substrate carrier 116 up and down in order to load and unload a substrate onto and out of the substrate carrier 116. This substrate carrier 116 is linked to the substrate carrier driver 118.
The process gas inside the reaction chamber 114 travels through the space between the reactor 100 and the substrate carrier 116 and then exhausted through the gas outlet tube 122. In this prior art, the gas outlet tube 122 and the gas inlet tube 120 are arranged asymmetrically with respect to the substrate 110, that is, the gas flow is imbalanced, thereby the flow of the process gas is biased towards the gas outlet tube 122.
Therefore, when a thin film is deposited on a substrate 110, if the flow of process gas is shifted forwards one side within the reaction chamber 114, the process gas is supplied unevenly over a substrate 110, thereby there is a tendency of forming a thinner film over a substrate 110 where there is less supply of process gas.
Accordingly, in order to minimize the imbalanced flow of process gas in the reactor 100, either the gas outlet tube must be preferably moved away from the substrate 110 or a means of even flow of process gas over the substrate 110 or the outflow tube 122 must preferably be relocated in such a way that the condition of flow of process gas over the substrate 110 is satisfied. However, the gas outlet tube 122 is relocated away from the substrate 110, the volume of the reaction chamber 114 increases, thereby the process cost increases due to the fact that the amount of gas for the same process step increases, and furthermore there is a difficulty of increased process time for supplying and removing gases or replacing process gases for the process of sequentially supplying various types of process gases. Therefore, the process time increases for processing ALD(Atomic Layer Deposition) method.
Further, in the example of this prior art, the substrate carrier 116 and the substrate carrier driver 118 are exposed to the process gas that is exhausted through the gas outlet tube 122, thereby undesirable thin film is deposited on or around the aforementioned parts. Later such undesirably deposited layer of thin film becomes not only the cause of undesirable micro-particles(contaminants) detrimental to the subsequent processing steps, but also the cause of potentially erratic operation of the parts inside the reaction chamber 114.
The second prior art shown in FIG. 1b, is to solve the problems in the first prior art shown in FIG. 1a as described above, and in FIG. 1b, the gas outlet tube 222 is positioned in the lower part of the middle of the reactor 200 so that the gas inlet tube 220 and the gas outlet tube 222 are arranged symmetrically. Specifically, referring to the schematic drawing of the second prior art as shown in FIG. 1b, unlike in FIG. 1a, the flow of process gas in the reaction chamber 214 is symmetric and even over the substrate 210, but the substrate carrier 216 and the substrate carrier drive 218 are still exposed to the process gas. Also, the volume of the reaction chamber can not be reduced significantly enough, in order to reduce the processing time for ALD processes. This is because the substrate carrier driver 218 still requires a certain minimum volume for moving the substrate carrier for normal operation of loading and unloading a substrate.
FIG. 2 is a timing sequence illustrating the process of conventional ALD process method. The vertical axis represents the process gas volume and the horizontal axis represents the processing time.
Referring to FIG. 2, the ALD process cycle is in the sequence of supply of first source gas 310→purge 312→supply of second source gas 314→purge 312. At the purge stage, the source gas supplied to the reaction chamber is removed either by evacuating the reaction chamber using a vacuum pump or by feeding an inert purge gas into the reaction chamber.
In the conventional ALD process, in case that the source gases react with each other easily, even a small amount of residual source gases may cause the undesirable generation of particles(contaminants), and therefore, it may be necessary to extend the time of purging. On the other hard, in case that the source gases react slowly, and as a result the processing time becomes longer, then the time for supplying the source gases may have to be lengthened, thereby the deposition time becomes longer.
Therefore, in order to solve these problems described above, the invertors of the present invention recently disclosed in KR0273473 a plasma enhanced atomic layer deposition method(PEALD), that improves the reactivity, reduces the purging time and thus improves the deposition rate, and as a result, improves the productivity of the deposition apparatus.
The difference between the plasma enhanced atomic layer deposition method and the aforementioned conventional ALD is a faster deposition rate even though source gases with low reactivity are used.
In the conventional ALD method aforementioned, when the source gases with low reaction rate are used, there is a slow reaction on the substrate, thereby, there may be a problem of lack of deposition of thin film.
However, the plasma enhanced atomic layer deposition method disclosed here increases the deposition reaction rate by generating radicals and ions with high reactivity by using a plasma, thereby these radicals and ions actively participate in the reaction process.
As an example of an apparatus that is suitable for processing using conventional ALD method, especially, the plasma enhanced ALD method has been disclosed in the patent application KR99-23078, “Chemical Deposition Reactor”, which invention reduces the volume of the reaction chamber, provides a means of even flow of the source gases when these gases supplied, and removed through the gas inlet tube and the gas outlet tube, respectively, flows over the substrate, and provided a means of generating a plasma. However, when this apparatus is used for forming a conductive thin film, a plasma may not be generated because of the fact that two electrodes used for applying RF power for plasma generation are electrically shorted due to the formation of a conductive thin film, and therefore, a conductive thin film may not be formed by using this apparatus along with the plasma enhanced atomic layer deposition method disclosed in the present invention.